Scavengers are historically employed with fuels that are combusted in internal combustion engines; in particular with fuels that already contain organometallic additives. The intent of adding scavengers is to mitigate or preferably eliminate any deleterious effects of the organometallic already used in the fuel including fouling and deposits formed in the engine.
Acknowledging the potent effects of organolead compounds as octane enhancers and antiknock additives, the piston engine aviation industry incorporated these compounds into aviation gasoline. Although organolead compounds provide significant benefits to aircraft piston engines in terms of octane rating enhancement, the lead deposits that form upon combustion are known to have deleterious effects on engine operability. In particular, the aviation industry is well aware of the propensity of lead deposits to foul piston engine spark plugs and cause misfiring. To ameliorate some of the negative aspects of combustion of organolead additives in internal combustion engines, lead scavengers have been incorporated into aviation gasoline. However, the aviation industry now seeks the removal of lead from aviation gasoline. The development of unleaded aviation gasoline that meets the industry standards for engine performance and operability remains a technologically challenging problem.
Replacing organolead antiknock additives with organornanganese compounds is a viable and promising solution. In one example, an organometallic manganese compound, specifically methylcyclopentadienyl manganese tricarbonyl (MMT), is employed as an octane booster. With these fuels that contain MMT, it is then desirable to employ a manganese scavenger to reduce or prevent fouling and deposit formation caused during the combustion of that fuel. Organobromine and organochlorine compounds, which are the most common lead scavengers, are believed relatively ineffective with manganese containing fuels. Instead, it is generally believed that phosphorous compounds are the most effective and commercially viable manganese scavengers. Unfortunately, it is known in the industry that phosphorous-containing scavengers can reduce the Motor Octane Number (MON) of a fuel containing organometallic antiknock compounds, including for instance the manganese-containing antiknock compounds. The mechanism of action is believed to be an antagonistic effect between the organometallic antiknock compounds and the scavenger that reduces the MON enhancing effect of the organometallic compound. This antagonistic effect on octane rating is significant enough to eliminate the practicality of an aviation gasoline containing manganese antiknock compounds.
Based on the prior art, tricresyl phosphate (TCP) is a well-known phosphorous based lead and manganese scavenger. However, TCP can reduce octane (MON) to unacceptable levels, as shown for instance in Example 1 below. Because of the challenge of meeting the high octane requirement, currently, of at least 99.6 Motor Octane Number (based on ASTM D-910) for aviation gasoline, even a small improvement in the antiknock effectiveness is significant. Thus the decrease in Motor Octane Number observed when employing TCP is significant enough to limit the commercialization of unleaded aviation gasoline containing organomanganese antiknocks that include TCP. Example 1 further shows some other phosphates that have similar or in fact worse impact on the MON.
The discovery detailed below describes the application of preferably phosphites and more preferably phosphines as manganese scavengers that limit the MON loss in aviation gasoline containing manganese antiknock compounds.